This invention relates to a polymer devolatilization apparatus comprising a flat plate heat exchanger and a related process for the devolatilization of polymer solutions at relatively high product flow rates.
The removal of volatile components from a polymer solution, referred to as "devolatilization", is a necessary step in the commercial manufacture of many polymers. In particular, where a polymer is produced from a solution of monomers, it is necessary to remove the solvent and unreacted monomers from the final product. For example, residual monomer and volatiles must be removed from the polymer product in the bulk or solution polymerization of polystyrene, styrene/acrylonitrile copolymers (SAN) or rubber modified styrene/acrylonitrile copolymers (ABS, AES, etc.).
The separation of the volatile components from the polymer is generally achieved by evaporation, the process consisting of heating the polymer solution at a temperature higher than its boiling point and removing the vapors formed. One method of devolatilization involves passing the polymer solution through a heat exchanger and then into a zone of reduced pressure. Suitable heat exchangers for this purpose, referred to as flat plate heaters or flat plate heat exchangers comprise a multiplicity of heated flat plates arranged in layers to leave channels connecting the interior to which a polymer solution is supplied and exterior portions of the heater for passage of the solution to be heated and devolatilized. Improved performance is attained by placing the heater within a closed shell which is partially evacuated. Previous designs of flat plate heaters have been disclosed in U.S. Pat. Nos. 3,014,702, 4,153,501, 4,421,162, 4,423,767, 4,564,063, 4,808,262, and 5,084,134, the teachings of which are hereby incorporated by reference.
More efficient designs of flat plate heaters use extended length channels and operate at lower temperatures which tend to improve the distribution of polymer solution through the heating channels. However, the use of channels longer than about 152 mm or 6 inches, can result in flow instability, particularly when polystyrene, SAN, ABS or AES polymer solutions are devolatilized. Flow instability has been shown to be a function of the flow rate of the polymer through the heated channels, and can be detected by the presence of pressure oscillations at the entrance of the channels. The magnitude of these oscillations becomes larger as the flow rate increases. Observations of such flow instability have been made by Maffetone, et al., "Slit Devolatilization of Polymers" AIChE Journal, 37, 724-34 (May 1991).
It would be desirable if there were provided an improved polymer devolatilization apparatus incorporating a flat plate heat exchanger having an improved heating channel design, which would allow high product flow rates, without the occurrence of significant pressure oscillations which cause flow instability.